1. Field of the Invention
The present invention relates to an objective lens used in an optical pickup for an optical disk.
2. Description of the Related Art
In general, coherent monochromatic light such as a laser beam is used to record, reproduce and erase information of high density with respect to an optical disk.
Accordingly, a normal objective lens for the optical disk is corrected with respect to aberration about a single wavelength, but is not sufficiently considered with respect to chromatic aberration.
A semiconductor laser has been recently used as a light source of an optical pickup. However, in the semiconductor laser, a wavelength of light is rapidly changed by a so-called mode hopping. When such a change in wavelength is caused, defocus is caused when no chromatic aberration is sufficiently considered with respect to the objective lens.
The change in wavelength by the mode hopping is caused by a change in atmospheric temperature and is also caused when a recording or erasing mode and a reproducing mode are switched.
In the optical pickup, focusing control is performed by a voice coil actuator, etc. When the change in wavelength by the mode hopping is caused, a focusing control speed cannot follow the defocusing operation caused by the change in wavelength so that it is difficult to record and reproduce data at a high speed.
Further, it is necessary to secure a sufficient working distance to prevent the objective lens from coming in contact with the optical disk when no focusing control is completed and an error in focusing operation is caused.